The present invention relates generally to a magnetic resonance imaging (MRI), and more particularly, to a method and apparatus for reduction of RF induced power in high field imaging using a modulated pulse sequence.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned, moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx Gy and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
High field imaging can theoretically provide a significant improvement in signal-to-noise ratio (SNR) compared to imaging at lower field strengths, but it is also burdened by a quadratic increase in peak and integrated RF power as a function of field strength. Therefore, at field strengths greater than 1.5 Tesla, induced RF power is a critical constraint in pulse sequence design. That is, peak power capacity of the system limits how compact a given RF pulse can be made. Further, limitations on average power induced in the patient further limits how quickly pulses can be repeated and thus, how much coverage can be attained in a given amount of time.
Certain pulse sequences, such as fast spin echo sequences, are designed to operate at or near regulatory power limits at low field strengths and therefore require particular challenges for use in high field strength to limit the amount of induced RF power. Particularly, standard 180° pulses that are used for refocusing must be limited significantly, and the advantages of high field imaging can thereby be lost.
Some common approaches to reduce RF power include using a reduced flip angle, or linearly stretching the pulses.
In the latter, RF pulses are reshaped to reduce power because the energy of an individual pulse is proportional to the integral of the square of the nutation rate, thereby reducing the bandwidth by linearly stretching the pulse as a means to reduce power while conserving total nutation angle. However, increasing the echo spacing also causes increased modulation due to relaxation. In other words, stretching RF pulses can result in longer echo spacing, but then exacerbate the effects of relaxation through the echo train, thereby resulting in degraded image quality.
A more advanced solution has been termed Variable Rate Selective Excitation (VERSE). In this technique, an RF pulse of a given nutation, duration, and bandwidth is designed by conventional means (such as a windowed sinc or optimized Shinnar-Le-Roux design), then reshaped such that high amplitude portions of the original RE pulse are stretched at a different rate than low amplitude portions of the pulse. The reshaped pulse has lower peak and total power than the original. However, this technique still suffers reduced image quality because it is sensitive to off-resonance and therefore produces weakened signals that reduce SNR in the presence of off-resonance.
Another method of reducing power in fast spin echo sequences is to reduce the nutation angle of the refocusing pulse train. While nutation angles much less than 180° can be used to generate an echo train, the signals produced oscillate and are reduced in amplitude. Additionally, reducing refocusing flip angles produces signal from stimulated echo pathways which have mixed T1 and T2 contrast as well as lower overall signal levels. The mixing of stimulated and spin echo signals result in signal decay, and thus image contrast, which has a T1 dependence as well as T2.
The problem of signal reduction and oscillation produced by low flip angle refocusing trains has been addressed by systematically varying the flip angles of the first few refocusing pulses. Such techniques allow an efficient transition of the spin system to a pseudo-steady-state which produces significantly higher signal than constant angle trains. It has also been demonstrated that by slowly varying the flip angle through the pulse train, a pseudo-steady-state condition can be maintained while manipulating the signal level received in different regions of k-space. By increasing the flip angle for acquisition of high k-space views, relaxation effects can be mitigated, and thus resolution improved. By increasing the flip angle for acquisition of low k-space views, SNR can be improved, however this also has the detrimental effect of increasing blurring.
In each of the aforementioned techniques, low angle refocusing flip angles generate significant contributions to signal from stimulated echo pathways, and this in turn, alters image contrast at a given effective echo time.
It would therefore be advantageous to have a technique capable of meeting the challenge of providing adequate coverage at high field strengths with reduced induced RF power that does not adversely affect signal, contrast, and resolution.